Injection Context

The inject() function can only be used when code is executing within an injection context.

Where is an Injection Context Available?

An injection context is automatically available in:

1. Field initializers of classes instantiated by DI (@Injectable, @Component, @Directive, @Pipe).
2. Constructors of classes instantiated by DI.
3. Factory functions specified in useFactory or InjectionToken configurations.
4. Functional APIs executed by Angular (e.g., functional route guards, resolvers, interceptors).

@Component({...})
export class Example {
  // Valid: Field initializer
  private router = inject(Router);

  constructor() {
    // Valid: Constructor
    const http = inject(HttpClient);
  }

  onClick() {
    // Invalid: Not an injection context
    // const auth = inject(AuthService);
  }
}

runInInjectionContext

If you need to run a function within an injection context (often needed for dynamic component creation or testing), use runInInjectionContext. This requires access to an existing injector (like EnvironmentInjector or Injector).

import {Injectable, inject, EnvironmentInjector, runInInjectionContext} from '@angular/core';

@Injectable({providedIn: 'root'})
export class MyService {
  private injector = inject(EnvironmentInjector);

  doSomethingDynamic() {
    runInInjectionContext(this.injector, () => {
      // Now valid to use inject() here
      const router = inject(Router);
    });
  }
}

assertInInjectionContext

Use assertInInjectionContext in utility functions to guarantee they are called from a valid context. It throws a clear error if not.

import {assertInInjectionContext, inject, ElementRef} from '@angular/core';

export function injectNativeElement<T extends Element>(): T {
  assertInInjectionContext(injectNativeElement);
  return inject(ElementRef).nativeElement;
}